Conventional melt delivery system used for compression molding articles such as closures comprises mixing a resin along with metered quantities of colorant and additives and feeding them into the resin hopper, and subsequently extruding the resin. At the exit to the extruder, the melt is metered by a melt pump, providing a controlled volume of extrudate out of the melt pump. If the molds to which the charges of resin are in a remote position, the extrudate passes through a hose or conduit to a nozzle. As the extrudate emerges from the nozzle, it is cut by a rotating blade into individual charges and deposited into successive mold cavities as they are conveyed beneath the cutter blade.
Such a process has previously been unsuccessful in the molding resin containing metallic pigment additives due to the formation of an aesthetically objectionable pellet blemish on the surface of the article. For example, when the article is a closure having a base wall and a peripheral skirt, with threads on a surface thereof, such a blemish is readily noticed on the base wall.
It was found that the above described delivery system causes the metallic flakes in the pigment to align in the direction of flow through the hose and nozzle such that they are parallel with the axis of the cylindrical shaped pellet at it exists the nozzle. During the cut off of the pellet, this alignment remains largely undistributed, such that at the juncture of the cylindrical sides of the pellet and the circular ends of the pellet, there is an abrupt change in alignment of the metallic flakes. It was found that this change in alignment is a major contributor of the pellet blemish effect, whereby the pellet is deposited in the cool cavity in a fairly random way, and the flake alignment becomes "frozen in" as the "skin" solidifies during forming against the cool cavity.
The material flow action during compression molding differs from that of injection molding. In compression molding, the pellet is placed in contact with the surface of the open mold and remains in this initial position for the period of time required to close the mold. Only during the final stages of the closing of the mold does the pellet begin to be deformed and to be displaced to fill the mold. At this time, the material within the pellet is flowing, whereas the initial contact or "skin" area of pellet remains essentially in its original position. This differs from injection molding where the material flows in from a central gate of small diameter, and immediately flows radially outward into a fully closed mold. This flow in injection molding results in alignment of the flakes as the melt is forced through the thin sections of the closed mold immediately. Alignment of the flakes parallel to the closure wall results in a good metallic appearance.
In U.S. Pat. No. 5,328,650, there is disclosed a method of removing the blemishes which comprises adding a blowing agent to the resin prior to extrusion, controlling the temperature of said melt such that the blowing agent is activated just prior to extrusion of the melt from the nozzle, the amount of blowing agent being sufficient to randomized the particle pigment additive such that gaseous bubbles are formed in the extruded melt in an amount sufficient to substantially reduce blemishes normally occurring in compression molding of resins containing particle pigment additive and in an amount not so great that the gaseous bubbles will not be substantially expelled from the melt during the compression molding to form the closure.
Among the objectives of the present invention is to provide a method of avoiding the blemishes in compression molded closures without the use of blowing agents or control of temperature of the resin.
In accordance with the invention, the particles added for pearlescence comprise spherical or ellipsoidal particles, such as aluminum particles, instead of flat pearlescent particles. The resultant compression molded closures have no discernable blemishes. Preferably the spherical or ellipsoidal particles are provided in a plastic carrier compatible with the resin of which the closure is made and formed into pellets which are then mixed with the resin to be extruded. A preferred resin comprises polypropylene for the closure and the carrier. When the molten blend of the resin and carrier resin with the ellipsoidal or spherical particles is extruded, cut off into pellets and compression molded in a cavity to form a closure, the resultant closure does not have any discernable blemishes.